docs: def Churchill_1977(Re, eD):
      r'''Calculates Darcy friction factor using the method in Churchill and
      (1977) [2]_ as shown in [1]_.
  
      .. math::
          f_f = 2\left[(\frac{8}{Re})^{12} + (A_2 + A_3)^{-1.5}\right]^{1/12}
  
      .. math::
          A_2 = \left\{2.457\ln\left[(\frac{7}{Re})^{0.9}
          + 0.27\frac{\epsilon}{D}\right]\right\}^{16}
  
      .. math::
          A_3 = \left( \frac{37530}{Re}\right)^{16}
  
      Parameters
      ----------
      Re : float
          Reynolds number, [-]
      eD : float
          Relative roughness, [-]
  
      Returns
      -------
      fd : float
          Darcy friction factor [-]
  
      Notes
      -----
      No range of validity specified for this equation.
  
      Examples
      --------
      >>> Churchill_1977(1E5, 1E-4)
      0.018462624566280075
  
      References
      ----------
      .. [1] Winning, H. and T. Coole. "Explicit Friction Factor Accuracy and
         Computational Efficiency for Turbulent Flow in Pipes." Flow, Turbulence
         and Combustion 90, no. 1 (January 1, 2013): 1-27.
         doi:10.1007/s10494-012-9419-7
      .. [2]	Churchill, S.W.: Friction factor equation spans all fluid flow
         regimes. Chem. Eng. J. 91, 91-92 (1977)
      '''
      A3 = (37530/Re)**16
      A2 = (2.457*log((7./Re)**0.9 + 0.27*eD))**16
      ff = 2*((8/Re)**12 + 1/(A2+A3)**1.5)**(1/12.)
      return 4*ff